In recent years, semiconductor devices have become more integrated, and structures of semiconductor elements have become more complicated. Further, a number of layers in multilayer interconnects used for a logical system has been increased. Accordingly, irregularities on a surface of a semiconductor device are increased, so that step heights on the surface of the semiconductor device tend to be large. This is because, in a manufacturing process of a semiconductor device, a thin film is formed on a semiconductor device, then micromachining processes, such as patterning or forming holes, are performed on the semiconductor device, and these processes are repeated to form subsequent thin films on the semiconductor device.
When a number of irregularities is increased on a surface of a semiconductor device, the following problems arise. When a thin film is formed on a semiconductor device, a thickness of the film formed at portions having a step is relatively small. Further, an open circuit may be caused by disconnection of interconnects, or a short circuit may be caused by insufficient insulation between interconnect layers. As a result, good products cannot be obtained, and a yield tends to be lowered. Further, even if a semiconductor device initially works normally, reliability of the semiconductor device is lowered after a long-term use. At a time of exposure in a lithography process, if an irradiation surface has irregularities, then a lens unit in an exposure system is locally unfocused. Therefore, if the irregularities of the surface of the semiconductor device are increased, then it becomes problematically difficult to form a fine pattern itself on the semiconductor device.
Thus, in a manufacturing process of a semiconductor device, it becomes increasingly important to planarize a surface of the semiconductor device. The most important one of planarizing technologies is chemical mechanical polishing (CMP). In a polishing apparatus for chemical mechanical polishing, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface, so that the substrate is polished.
This type of polishing apparatus includes a polishing table having a polishing surface formed by a polishing pad, and a substrate holding device, which is referred to as a top ring or a carrier head, for holding a substrate such as a semiconductor wafer. When a semiconductor wafer is polished with such a polishing apparatus, the semiconductor wafer is held and pressed against the polishing surface under a predetermined pressure by the substrate holding device. At that time, the polishing table and the substrate holding device are moved relative to each other to bring the semiconductor wafer into sliding contact with the polishing surface, so that a surface of the semiconductor wafer is polished to a flat mirror finish.
In such a polishing apparatus, if a relative pressing force between the semiconductor wafer being polished and the polishing surface of the polishing pad is not uniform over an entire surface of the semiconductor wafer, then the semiconductor wafer may be insufficiently polished or may be excessively polished at some portions depending on a pressing force applied to those portions of the semiconductor wafer. Therefore, it has been attempted to form a surface, for holding a semiconductor wafer, of a substrate holding device from a membrane made of an elastic material such as rubber and to supply fluid pressure such as air pressure to a backside surface of the membrane to uniformize pressing forces applied to the semiconductor wafer over an entire surface of the semiconductor wafer.
If a transfer device such as a robot is used to directly deliver a semiconductor wafer to be polished to the substrate holding device and directly receive a polished semiconductor wafer from the substrate holding device, then the transfer device may fail in this transfer because of a difference of transferring precision between the transfer device and the substrate holding device. Accordingly, the polishing apparatus may include a substrate relay device disposed at a delivery position of a semiconductor wafer to the substrate holding device or at a receiving position of a semiconductor wafer from the substrate holding device. Such a substrate relay device is referred to as a pusher. A semiconductor wafer transferred by a transfer device such as a robot is placed on the substrate relay device. Then, the substrate relay device lifts the semiconductor wafer to the substrate holding device such as a top ring, which has been moved above the substrate relay device, and delivers the semiconductor wafer to the substrate holding device. Further, the substrate relay device receives the semiconductor wafer from the substrate holding device and delivers the semiconductor wafer to the transfer device.
When a substrate such as a semiconductor wafer is transferred from a substrate holding device such as a top ring to a pusher (substrate relay device), a pressurized fluid (a gas, a liquid, or a mixture of a gas and a liquid) is introduced into a fluid passage provided in the top ring to eject and remove the semiconductor wafer from the top ring. At that time, since a gap is formed between the top ring and the pusher, the semiconductor wafer falls down through the gap after it is separated from the top ring. The pusher catches and receives this fallen semiconductor wafer.
In the aforementioned polishing apparatus, a semiconductor wafer is polished under various polishing conditions including a type of slurry (polishing liquid), polishing time, pressing forces of the semiconductor wafer, and rotational speeds of a top ring and a polishing table. Under some polishing conditions, a polished semiconductor wafer may firmly adhere to the top ring when the semiconductor wafer is to be separated from the top ring. In such a case, the semiconductor wafer cannot be removed from the top ring. Particularly, when a surface, for holding a semiconductor wafer, of a substrate holding device is formed by a membrane, and a fluid pressure such as air pressure is supplied to a backside surface of the membrane to press the semiconductor wafer against the polishing surface on the polishing table, the following problems may arise because the membrane is made of rubber. When the semiconductor wafer is to be separated from the substrate holding device after polishing, the semiconductor wafer adheres to the membrane so that it cannot be removed from the substrate holding device. Otherwise, it takes much time to separate the semiconductor wafer from the substrate holding device. Further, the semiconductor wafer may fall down in an oblique state while a portion of the semiconductor wafer adheres to the membrane. In such cases, if a pressure of fluid ejected from the top ring is increased in order to reliably remove the semiconductor wafer from the top ring, then the semiconductor wafer falls down toward the pusher with force, thereby causing damage or breakage of the semiconductor wafer.
In recent years, low-k materials having a low dielectric constant have been developed as interlayer dielectrics instead of SiO2. However, such low-k materials have a low mechanical strength and are thus likely to be broken. Accordingly, if a semiconductor wafer employing such a low-k material is to be removed from the top ring by ejection of a pressurized fluid, the low-k material in the semiconductor wafer is broken by impact of falling, so that a yield is lowered.